Conventionally, a three-way catalyst capable of simultaneously performing oxidation of carbon monoxide (CO) and hydrocarbon (HC) and reduction of nitrogen oxide (NOx) in an exhaust gas has been used as a catalyst for purifying an automotive exhaust gas. A catalyst prepared by supporting a platinum group element such as platinum (Pt), rhodium (Rh) and palladium (Pd) on a porous oxide support such as alumina (Al2O3) is widely known as a three-way catalyst. Among others, Rh has a high NOx reducing activity, and therefore is an essential component in an exhaust gas purification catalyst such as the three-way catalyst.
Patent Document 1 describes an exhaust gas treatment catalyst comprising a support and Ir and another metal supported thereon in an alloyed state. Further, Patent Document 1 describes that Rh is suitable as another metal for improving the low-temperature activity and high-temperature activity. In addition, Patent Document 1 describes that the supporting ratio of the another metal to Ir is preferably in the ratio which does not exceed 1:1 in terms of atomic ratio.
Patent Document 2 describes a process for the catalytic partial oxidation of a hydrocarbonaceous feedstock, comprising contacting a feed comprising the hydrocarbonaceous feedstock and an oxygen-containing gas with a catalyst comprising metals of Group VIII of the Periodic Table of Elements, wherein the Group VIII metals are at least rhodium and iridium in intimate association with each other.
Non-Patent Document 1 relates to Ir that is a platinum group element similarly to Rh, where an iridium catalyst such as Ir/SiO2 obtained by supporting the element alone on a catalyst support is studied. Non-Patent Document 1 describes that even under oxidative reaction conditions, the surface of Ir particles is partially reduced to an Irδ+ surface species allowing adsorption of CO and/or NO.